1. Field of the Invention
The invention relates to millimeter wave beams and more particularly to a switch that either reflects or is transparent to a millimeter beam.
2. Description of the Related Art
Electromagnetic signals are commonly guided from a radiating element to a destination via a coaxial cable or metal waveguide. As the frequency of the signal increases, the coaxial cable or metal waveguide used to guide the signals have smaller cross-sections. For example, a metal waveguide that is 58.420 cm wide and 29.210 high at its inside dimensions, transmits signals in the range of 0.32 to 0.49 GHz. A metal waveguide that is 0.711 cm wide and 0.356 cm high at its inside dimensions, transmits signals in the range of 26.40 to 40.00 GHz. [Dorf, The Electrical Engineering Handbook, Second Edition, Section 37.2, Page 946 (1997)]. As the signal frequencies continue to increase a point is reached where the coaxial cables and waveguides become impractical. They become too small and expensive and require precision machining to produce. In addition, their insertion can become too great.
High frequency signals in the range of approximately 1 to 50 GHz, can be guided through a microstrip transmission line. However, at frequencies above this range, the microstrip suffers from the same problems; the transmission line becomes too small and the insertion loss from transmission through the line becomes too great.
Frequencies exceeding approximately 100 GHz (referred to as millimeter waves) should not be transmitted over a distance by a microstrip transmission line because of the insertion loss. Instead, the signal can be transmitted as a free-space beam. The signal from a radiating element is directed to a lens that focuses the signal into a millimeter wave beam having a diameter up to several centimeters. The beam is transmitted to a receiving lens that focuses the signal to a receiving element which often includes an amplifier. This form of transmission is referred to as xe2x80x9cquasi-opticxe2x80x9d when the lens diameter divided by the signal wavelength is in the range of approximately 1-10. In the optic regime, the lens diameter divided by the frequency wavelength is normally much greater than 10. [IEEE Press, Paul f. Goldsmith, Quasi-optic Systems, Chapter 1, Gaussian Beam Propagation and Applications (1999)]
For quasi-optic or optic transmission in military or commercial applications, a safety mechanism is normally needed in the beams path in the form of a shutter that either blocks the beam from reaching the component that needs protection, or allows the beam to reach the component. The mechanism is primarily used to protect delicate amplifiers at the receiving end of the transmission line from power surges at the radiating element. Mechanical shutters have been used for this purpose, but they are generally too slow at blocking the beam and are too unreliable because of complex mechanical components.
Another important characteristic of transmission in metal waveguides is the transmission cut-off frequency. If the frequency of the transmitted signal is above the cut-off frequency, the electromagnetic energy can be transmitted through the guide with minimal attenuation. Electromagnetic energy with a frequency below the cut-off will be totally reflected at entry to the guide and will be attenuated to a negligible value in a relatively short distance through the waveguide. The physical dimensions of a metal waveguide not only determines the range of frequencies that it transmits, but also the cut-off frequency for the fundamental (first) mode. The two waveguides described above have cut-off frequencies of 0.257 GHz and 21.097 GHz, respectively.
A structure has been developed that presents as a high impedance to transverse E fields of electromagnetic signals. [M. Kim et al., A Rectangular TEM Waveguide with Photonic Crystal Walls for Excitation of Quasi-Optic Amplifiers, (1999) IEEE MTT-S, Archived on CDROM]. The structure is particularly applicable to the sidewalls and/or top and bottom walls of metal rectangular waveguides. Either two or four of the waveguide""s walls can have this structure, depending upon the polarizations of the signal being transmitted. The structure comprises a substrate of dielectric material with parallel strips of conductive material that are separated by small (capacitive) gaps. It also includes inductive metal vias through the sheet to a conductive sheet on the substrate""s surface opposite the strips. At a certain frequency the inductance of the vias and the capacitance of the gaps resonate. At this xe2x80x9cresonantxe2x80x9d frequency, the surface impedance of becomes very high.
When used on a rectangular waveguide""s sidewalls, the structure provides a high impedance boundary condition for the E field component of a fundamental mode vertically polarized signal, the E field being transverse to the conductive strips. The high impedance prevents the E field from dropping off near the waveguide""s sidewalls, maintaining an E field of uniform density across the waveguide""s cross-section. Current can flow down the waveguide""s conductive top and bottom walls to support the signal""s H field with uniform density. Accordingly, the signal maintains near uniform power density across the waveguide aperture.
When the high impedance structure is used on all four of the waveguide""s walls, the waveguide can transmit independent cross-polarized signals each one being similar to a free-space wave having a near-uniform power density. The structure on the waveguide""s sidewalls presents a high impedance to the E field of the vertically polarized signal, while the structure on the waveguide""s top and bottom walls presents a high impedance to the horizontally polarized signal. The structure also allows conduction through the strips to support the signal""s H field component of both polarizations. Thus, a cross-polarized signal of uniform density can be transmitted.
Waveguides employing these high impedance structures are also able to transmit signals close to the resonant frequency that would otherwise be cut-off because of the waveguide""s dimensions if all of the waveguide""s walls were conductive. At resonant frequency, the waveguide essentially has no cut-off frequency and can support uniform density signals when its width is reduced well below the width for which the frequency being transmitted would be cut-off in a metal waveguide.
The present invention provides a new millimeter beam shutter switch that is placed in a millimeter beams path and is either opaque and blocks the beam, or is transparent and allows the beam to pass with minimal attenuation. The new switch can change states between opaque and transparent in microseconds or less without employing complicated or unreliable mechanical components.
The new shutter switch includes a plurality of waveguides adapted to receive at least part of the electromagnetic beam. The waveguides are adjacent to one another with their longitudinal axes aligned with the propagation of the beam. The waveguides switchable to either transmit or block the transmission of their respective portions of the beam.
The new shutter switch uses rectangular waveguides with high impedance structures on at least two opposing interior walls. The high impedance structures allow smaller waveguides to transmit signals that would otherwise be cutoff if all of the waveguide""s walls were conductive. The cross-section of each individual waveguide can be smaller than the beam""s cross-section, and the shutter switch includes a sufficient number of waveguides to intercept the entire beam. The waveguides are mounted adjacent to one another to form a wall, with each of the waveguide""s longitudinal axes aligned with the millimeter beam""s propagation axis. Each of the high impedance structures has shorting switches that, when closed, cause the structure to change from a high impedance surface to a conductive surface.
One embodiment of the shutter switch uses waveguides that have high impedance structures on their sidewalls, which allows each of the waveguides to transmit uniform density, vertically polarized signals at a particular design frequency. The preferred high impedance sidewalls comprise a sheet of dielectric material with a conductive layer on one side. The opposite side of the dielectric material has a series of parallel conductive strips that are oriented down the waveguide""s longitudinal axis. Each of the strips has a uniform width, with uniform gaps between adjacent strips. Vias of conductive material are provided through the dielectric material between the conductive layer and the conductive strips. The actual dimensions of the surface structure depend on the materials used and the signal frequency.
During transmission of a vertically polarized signal, the waveguide carries an E field component transverse to the surface structure""s conductive strips. At a design frequency, the vias which extend through the substrate present an inductive reactance (2xcfx80fL), while the gaps between the strips present an approximately equal capacitive reactance (1/(2xcfx80fC)). The surface presents parallel resonant L-C circuits to the transverse E field component; i.e. a high impedance. The L-C circuits present an open-circuit to the transverse E-field, allowing it to remain uniform across the waveguide. The low impedance on the top and bottom waveguide walls allows current to flow and maintains a uniform H field. Each of the waveguides transmits the signal with uniform density, and the shutter switch appears transparent to the vertically polarized beams at the design frequency.
When the shorting switches on the high impedance structure are closed, the high impedance sidewalls are switched to a conductive surface. All of the waveguide""s walls become conductive and, because of the waveguide""s dimensions, signal transmission is cut-off. If the shorting switches are closed in all of the shutter switch""s waveguides, transmission is blocked in all the waveguides and the shutter switch becomes opaque to the beam. Similarly, if the shutter switch has waveguides with the high impedance structure on the top and bottom walls, the shutter switch could be used to block or transmit horizontally polarized signals.
In another embodiment of the waveguide used to form a shutter switch, the high impedance structure is placed on all four of the waveguides walls. This allows the waveguide to transmit a cross-polarized signal (vertical and horizontal) at a particular resonant frequency. When the shorting switches are closed on the high impedance structure in all the waveguides, the shutter switch blocks transmission of the cross-polarized signal. The shorting switches can also be selectively closed to block transmission of only one polarization of the cross polarized signal. Closing the shorting switches on the waveguide""s sidewalls blocks the vertically polarized signal, while closing the shorting switches on the top and bottom walls blocks the horizontally signal.
In still another embodiment, either two or all four of the waveguides sidewalls have a multi-layered high impedance structure which causes each of the layers to present a high impedance to a transverse E field at widely separated resonant frequencies. The number of frequencies that the waveguide can transmit with uniform density depends on the number of layers in the structure. When the multi-layered structure is on the sidewalls only, the waveguide transmits vertically polarized signals; when the multi-layered structure on the top and bottom walls, the waveguide transmits horizontally polarized signals. When the multi-layered structure is on all four of the waveguide""s wall, the waveguide can transmit either a single polarized signal or both cross-polarized signals. Shorting switches on the multi-layered structures can be selectively closed to block transmission of one or both of the polarizations, at one of the different transmission frequencies.
Different shorting switches can be used to switch the high impedance surface structures to a conductive surface. The preferred switches consume a relatively small amount of power and employ varactor layer diode technology or micro electromechanical system (MEMS) technology.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: